Go to the Nortel website, go to the support/downloads section, and search for the models you have. There will be a "MIBs & Router Scripts" download section that should display the latest MIBs - make sure you download the MIB that matches your OS rev. on the Nortel device.
Once you've DL'ed the MIB, the next steps depend on which OV app you are using - I assume NNM? If so, use the TOOLS mennu to locate the MIB installation utility.